Method for reducing wiring and required number of redundant elements

ABSTRACT

A method and enhanced Static Random Access Memory (SRAM) redundancy circuit reduce wiring and the required number of redundant elements. A bitline redundancy mechanism allows the swapping of a pair of bitlines for a redundant pair of bit columns. Two of the adjacent bitlines are swapped out at a time, one even and one odd. The swap is accomplished by steering the data around the bad columns and adding redundant columns on the end that are steered in when needed.

FIELD OF THE INVENTION

The present invention relates generally to the data processing field,and more particularly, relates to a method and enhanced Static RandomAccess Memory (SRAM) redundancy circuit to reduce wiring and therequired number of redundant elements.

DESCRIPTION OF THE RELATED ART

Referring to FIGS. 1, 2, and 3, a prior art SRAM redundancy arrangementis shown. This conventional SRAM redundancy arrangement can only swapout defective cells starting on an even column. This necessitated usingfour redundant columns labeled R in FIG. 1 to cover all requiredrepairs, including repairs between any two columns.

As shown in FIG. 1, the prior art SRAM redundancy circuit includes aplurality of pairs of series connected 2:1 multiplexers (MUXs). Eachpair of series connected MUXs is connected to an adjacent pair of oddand even SRAM columns. The first MUX of a pair of MUXs is a bit decodeMUX, each of the first MUXs receiving a select bit (BS). In operation,all the first MUXs take input A, the even SRAM column; or all take inputB, the odd SRAM column. With a BS=0, the A input is selected and BS=1,the B input is selected.

Redundancy steering is provided with the second MUX after the first bitdecode MUX in the prior art arrangement. Replacements are provided onlyon even bits, and that in turn forces the need for 4 redundant elements.For example, as indicated by an X between columns 1 and 2 in FIG. 1, ashift of four bits are needed for any 2 bit replacement, asschematically illustrated above the columns 2, 3, 4, and 5.

FIG. 2 shows a prior art predecoder block that feeds the prior art SRAMredundancy decode circuits shown in FIG. 3. The prior art SRAMredundancy decode circuits as shown in FIG. 3 include 64 redundantdecode circuits, one for each even SRAM column in a SRAM design with 128columns. A standard 2 to 4 address predecoder illustrated in FIG. 2 isused to generate the inputs to the first stage of the redundant decodecircuit. A redundant enable (RED-EN) is applied to a buffer thatprovides a redundancy decode signal RED_BL_MASTER. In each of the 64 bitdecode circuits, a steering input STEER_T_MINUS_1 is applied to aninverter 302. The predecoded bits labeled GROUP 01, GROUP 23, GROUP 45are applied to a 3-input NAND 304. The output of inverter 302 and NAND304 are applied to a NAND 306. The output of NAND 306 and the redundancydecode signal RED_BL_MASTER is applied to a NAND 308. NAND 308 generatesa STEER_C signal that is inverted by an inverter 310 generating aSTEER_T signal. In operation, the redundancy address decodes to thefirst column to be shifted or steered. Then each stage feeds the resultsof its decode to the next decoders 300, #0-63, such that once thedecoded one is found it forces all subsequent decodes to be true. Everycolumn after that one is then shifted and at the end the redundantelements are steered in.

In addition to requiring four redundant columns to cover all requiredrepairs the prior art arrangement, another disadvantage is that theredundancy decode signal RED_BL_MASTER requires a respective separatewire for connection to each of the decoders 300, #0-63.

SUMMARY OF THE INVENTION

Principal aspects of the present invention are to provide a method andenhanced Static Random Access Memory (SRAM) redundancy circuit to reducewiring and the required number of redundant elements. Other importantaspects of the present invention are to provide such method and enhancedStatic Random Access Memory (SRAM) redundancy circuit to reduce wiringand the required number of redundant elements substantially withoutnegative effect and that overcome some of the disadvantages of prior artarrangements.

In brief, a method and enhanced Static Random Access Memory (SRAM)redundancy circuit are provided to reduce wiring and the required numberof redundant elements. A bitline redundancy mechanism allows theswapping of a pair of bitlines for a redundant pair. The bit columns areinterleaved with respective even and odd bits, two of the adjacent bitsare swapped out at a time, one even and one odd. The swap isaccomplished by steering the data around the bad columns and addingredundant columns on the end that are steered in when needed.

In accordance with features of the invention, only two redundant columnsare required to replace a defect that includes two bad columns. With aSRAM including 128 columns, 128 redundant decode circuits are used. Anembedded redundant enable is applied to an input each of the 128redundant decode circuits and thereby an additional wire advantageouslyis saved by embedding the redundancy enable as compared to prior artarrangements.

In accordance with features of the invention, a single multiplexer (MUX)provides redundancy steering incorporated with bit decode. A bit select(BS) and a redundancy steer select (RS<0:1>) are combined to providesteering around a bad column and adding redundant columns in the SRAMredundancy circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention together with the above and other objects andadvantages may best be understood from the following detaileddescription of the preferred embodiments of the invention illustrated inthe drawings, wherein:

FIGS. 1, 2, and 3 illustrates a prior art SRAM redundancy arrangement;

FIG. 4 illustrates an exemplary SRAM redundancy circuit in accordancewith the preferred embodiment;

FIGS. 5A and 5B together illustrate an exemplary 4:1 multiplexer (MUX)of the SRAM redundancy circuit of FIG. 4 in accordance with thepreferred embodiment;

FIG. 6 illustrates exemplary SRAM redundancy decode circuits inaccordance with the preferred embodiment; and

FIGS. 7A and 7B together illustrate an exemplary 2:4 predecoder usedwith the SRAM redundancy decode circuits of FIG. 6 in accordance withthe preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with features of the invention, a method and enhancedStatic Random Access Memory (SRAM) redundancy circuit are provided toreduce wiring and the required number of redundant elements. A bitlineredundancy mechanism allows the swapping of a pair of bitlines for agood set. By enabling the decode to occur at any column, all requiredrepairs may be made with only two redundant columns. The bit columns areinterleaved with even and odd bits; two of the adjacent bits are swappedout at a time, one even and one odd. The swap is accomplished bysteering the data around the bad columns and adding redundant columns onthe end that are steered in when needed. A single multiplexer (MUX)provides redundancy steering incorporated with bit decode. In theredundancy steering bit decode of the invention, it is guaranteed thatonce steering is started, all bits thereafter are steered as required.

In accordance with features of the invention, only two redundant columnsare required to replace a defect including two defective columns.Assuming a SRAM design with 128 columns, the prior art requires 64redundant decode circuits, one for each even column. The invention hashigher granularity, using 128 redundant decode circuits. Both the priorart and the invention use standard 2 to 4 address predecoders togenerate the inputs to the first stage of the redundant decode circuit.The redundant address decodes to the first column to be shifted orsteered. Every column after that one is then shifted and at the end theredundant elements are steered in.

Having reference now to the drawings, in FIG. 4, there is shown anexemplary SRAM redundancy circuit generally designated by the referencecharacter 400 in accordance with the preferred embodiment. SRAMredundancy circuit 400 includes a plurality of SRAM columns 402including a pair of redundancy SRAM columns labeled R. A single 4:1multiplexer (MUX) 404 provides redundancy steering incorporated into abit decode. Each MUX 404 is used instead of the two 2:1 MUXs in seriesof the prior art arrangement of FIG. 1.

Each MUX 404 receives a select bit (BS) and the redundancy selectRS<0:1>. Each column 402 is fed into two of the 4:1 MUXs, with theexception of the first two columns 0, 1. The output from column 0, 402goes to A of the first MUX 404, column 1 goes to B of the first MUX 404,column 2 goes to C of first MUX 404 and to A of the second MUX 404,column 3 goes to D of first MUX 404 and B of second MUX 404, and thelike. Note that each 4:1 MUX 404 has 3 selector inputs (BS, RS<0> andRS<1>). BS is the same select bit as in prior art as shown FIG. 1.RS<0:1> are steering selects provided by the decoder circuitsillustrated in FIG. 6. For example, as indicated by an X between columns1 and 2 in FIG. 4, a shift of two bits are needed for any 2 bitreplacement, as schematically illustrated above the columns 1 and 2.

FIGS. 5A and 5B together illustrate an exemplary 4:1 multiplexer (MUX)404 of the SRAM redundancy circuit 400 of FIG. 4 in accordance with thepreferred embodiment.

Referring now to FIG. 5A, an element E, 500 is shown that is used toconstruct the 4:1 MUX 404. Element E, 500 includes a first inverter 502receiving an A input, and a second inverter 504 receiving an input S.Element E, 500 includes a parallel-connected P-channel field effecttransistor (PFET) 506 and N-channel field effect transistor (NFET) 508providing an output Z. Second inverter 504 provides a gate input to PFET506 and input S connects to a gate input of NFET 508.

Referring also to FIG. 5B, 4:1 MUX 404 includes a plurality of elementsE #1-4, 500 receiving a respective input A, B, C, D and the input S.Selector inputs BS, RS<0> and RS<1> are applied via logic functiondefined by a plurality of invertors 510, 512, 514, 516, and a pluralityof AND gates 520, 522, 524, 526. Input BS is applied to AND gates 522,526, and is inverted and applied to AND gates 520, 524. The redundancyselect RS<0> is applied to AND gate 524 and is inverted and applied toAND gate 520. The redundancy select RS<1> is applied to AND gate 526 andis inverted and applied to AND gate 522. A respective output S of ANDgates 520, 522, 524, 526 is applied to the input S of elements E #1-4,500. An output Z of respective elements E #1-4, 500 is applied to aninverter 530, providing anon-inverted output of the 4:1 MUX 404.

In accordance with features of the invention, the bit select (BS) andthe redundancy steer select (RS<0:1>) are combined to properly selectthe correct bit. Essentially, when a steering bit is present, the MUX404 takes the bit from two columns over and starts steering with everybit thereafter being steered, and two columns are never steered into thesame MUX output. SRAM redundancy circuit 400 with MUX 404 provides asignificant change from the prior art SRAM redundancy circuit of FIG. 1,which caused a shift of 4 bits for any 2 bit replacement.

Referring now to FIG. 6, there is shown an exemplary SRAM redundancydecode generally designated by the reference character 600 in accordancewith the preferred embodiment. SRAM redundancy decode 600 includes 128redundant decode circuits 602, #0-127. SRAM redundancy decode 600includes one redundant decode circuits 602 for each even and odd SRAMcolumn in a SRAM design with 128 columns. A standard 2 to 4 addresspredecoder illustrated in FIGS. 7A and 7B is used to generate the inputsto the redundant decode circuits 602.

An embedded redundant enable is applied to an input each of the 128redundant decode circuits 602. An additional wire advantageously issaved by embedding the redundancy enable into the decode 602, ascompared to the prior art decode circuits illustrated in FIG. 3.

Each of the 128 redundant decode circuits 602, #0-127 includes a pair ofNAND gates 604, 606 respectively receiving inputs A, B and C, D andhaving a respective output applied to an OR gate 608. The output of ORgate 608 and a decode signal SN_M1 is applied to a NAND 610. NAND 610generates a signal that is sequentially inverted by a pair of seriesconnected inverters 612, 614 to generate an output S signal. Theredundant address decodes to the first column to be shifted or steered.Then each stage 602, #0-127 feeds the results SN_M1 of its decode atnode TN at the output of the first inverter 612 to the next respectiveone of the decoders 602, #0-127.

FIGS. 7A and 7B together illustrate an exemplary 2:4 predecodergenerally designated by the reference character 700 for use with theSRAM redundancy decode 600 of FIG. 6 in accordance with the preferredembodiment. The 2:4 predecoder 700 receives a redundant enable signalRED-ED applied to an enable input EN.

Referring now to FIG. 7B, the 2:4 predecoder 700 includes a pair ofinverters 702, 704 respectively receiving the A input and the B input.The outputs of inverters 702, 704 and the A input and the B input areapplied to respective inputs of a plurality of 3-input NAND gates 706,708, 710, 712, as shown. The redundant enable input EN is applied to aninput of each of 3-input NAND gates 706, 708, 710, 712. The respectiveoutput of each of 3-input NAND gates 706, 708, 710, 712 is applied to arespective one of a plurality of inverters 714, 716, 718, 720 providingoutputs Z0, Z1, Z2, Z3.

While the present invention has been described with reference to thedetails of the embodiments of the invention shown in the drawing, thesedetails are not intended to limit the scope of the invention as claimedin the appended claims.

1. A method for implementing a Static Random Access Memory (SRAM)redundancy circuit with bit columns interleaved with respective even andodd bits comprising: providing a bitline redundancy mechanism to allowswapping of a pair of bitlines for a redundant bitline pair; swappingtwo of the adjacent bitlines at a time, one even and one odd by steeringaround at least one bad column and adding two redundant columns.
 2. Themethod for implementing a SRAM circuit as recited in claim 1 includesproviding a multiplexer for providing redundancy steering incorporatedwith a bit decode.
 3. the method for implementing a SRAM circuit asrecited in claim 2 includes combining a bit select (BS) and a redundancysteer select (RS<0:1>) to provide select input to said multiplexer. 4.The method for implementing a SRAM circuit as recited in claim 2includes applying an embedded redundant enable to an input of aredundancy decode circuit.